This invention relates to an improved process and electroplating bath for the electrodeposition of metal, and more particularly to an improved process and electroplating bath for the formation of electrodeposits of nickel and nickel alloys.
The use of coumarin as an additive in nickel electroplating baths, especially semi-bright nickel processes, to produce ductile, lustrous deposits with excellent leveling is well known. It is further known that the degree of leveling obtained is generally proportional to the concentration of coumarin in the plating bath. Thus, a high concentration of coumarin gives the best leveling. But such characteristics are short-lived since such high coumarin concentrations also result in a high rate of formation of detrimental breakdown or degradation products. These degradation products are objectionable in that they can cause uneven, dull gray areas which are not readily brightened by a subsequent bright nickel deposit; they can reduce the leveling obtained from a given concentration of coumarin in the plating bath; and they can reduce the beneficial physical properties of the nickel electrodeposits.
As noted above, the fact that coumarin breaks down or degrades under many conditions is well known. In the operation of plating baths containing coumarin, it is therefore usually necessary to monitor such degradation so that plating is not adversely affected. One method commonly used to monitor the degradation of such plating baths is a test method known as the "TF Index", where "TF" stands for treatment factor. The "TF Index" is a measure of the amount of coumarin degradation products present in such baths. Normally, melilotic acid is a primary degradation product found in plating baths containing coumarin, although other degradents are also present in smaller quantities. In general, a "TF Index" of from about 0.5 to about 2 indicates a tolerable level of degradation products, whereas a "TF Index" of over about 5 would indicate that the plating bath was probably not operating as desired and that the physical properties and appearance of the resulting plated materials would be unsatisfactory. In extreme cases, e.g., where insoluble anodes are used, "TF Indexes" as low as 1.5 to 2.0 have been known to indicate deleterious effects on the subsequent deposits. At this point, a batch treatment of the plating bath with activated carbon would be necessary to remove the degradents. Of course, such as batch carbon treatment requires that the plating batn and production be shut down. Needless to say, in addition to wasted production time and reduced output of plated parts, labor costs are incurred in conducting the batch carbon treatment. Also, new coumarin must be added to the plating bath, and the cost of such new coumarin is by no means negligible.
It has been known to reduce the concentration of coumarin in order to reduce degradation products and thereby increase bath life, but such reduction in coumarin concentration is usually accompanied by a loss in leveling and makes the bath more sensitive to degradent build-up. Also, the use of various additives such as aldehydes (including formaldehyde and chloral hydrate), has been proposed to help overcome the undesirable effects of the coumarin degradation products. The use of such additives has, however, had certain limitations since even moderate concentrations of these materials not only increase the tensile stress of the nickel electrodeposits but also appreciably reduce the leveling action of the coumarin. It has further been proposed to overcome the difficulty encountered in using coumarin as an additive in nickel plating baths by including in the baths an ethylene oxide adduct of an acetylenic compound. Although this technique has been helpful in overcoming the problems encountered in the use of coumarin, its beneficial effects are relatively short-lived.
U.S. Pat. No. 3,719,568 and U.S. Pat. No. 3,795,592 describe improvements whereby the use of specific ether adducts of propargyl alcohol, including propylene oxide adducts and propane sultone adducts, extend the life of coumarin based baths. This means the resultant baths do not have to be treated as often for degradent formation. Baths so treated also maintain the desired properties longer. Butyne diol is also mentioned in these patents as an additional additive which helps to maintain desired leveling characteristics. While these above-mentioned additives are indeed effective, such processes still have to be batch treated, using activated carbon, sometimes as often as every two or three weeks, depending on the nature of the installation. Further, while leveling is maintained at a higher degree than without these additives, such leveling still has a tendency to decrease with time, such that as the organic degradents build, the leveling still diminishes considerably.
It is also known that coumarin based processes normally provide poorer corrosion properties than other nickel processes. This is readily demonstrated in accelerated tests such as the conventional "CASS" and "Corrodkote" tests widely used in the plating arts. The use of additives such as aldehydes to increase corrosion resistance has only met with limited success. In addition to processes and plating baths having coumarin and various additives therein, similar efforts to develop suitable additives have been directed at oxyomegasulfohydrocarbon-di-yl coumarin, which in general does not produce leveled deposits when used alone, unless very high concentrations are used. Typical of plating processes and baths of the above-identified types, including both coumarin based and oxyomegasulfohydrocarbon-di-yl coumarin based processes and baths, are those described in United States Pat. Nos. 3,111,466; 3,367,854; 3,414,491; 3,556,959; 3,677,913; 3,719,568; and 3,795,592; to which reference is made for the further details of the processes, and the disclosures of which are incorporated by reference.
The present invention is believed to be applicable to coumarin based processes and baths of the foregoing type and is specifically directed to an improved process and bath which provides benefits and advantages heretofore unattainable with prior art practices. More particularly, it is a principle object of the present invention to provide a coumarin based process and electroplating bath which will run considerably longer than the processes described above, will sustain desired leveling characteristics, and will provide improved corrosion resistance.